Every Financial Deadline You Face Before December 31

October starts tomorrow, and with it comes a three-month stretch packed with financial deadlines that can cost you real money if you miss them. I'm not talking about small oversights. Missing your RMD deadline, for example, carries a 25% penalty. Forgetting to spend down your FSA means forfeiting hundreds of dollars. Letting open enrollment pass without reviewing your health plan could lock you into the wrong coverage for all of 2027.

I built this calendar because I got burned a few years ago. I missed a December 15 enrollment deadline by two days, stuck myself with a plan that didn't cover my specialist, and spent January on the phone trying to fix it. That was expensive and entirely avoidable.

Here's every date that matters between now and midnight on December 31, organized by month so you can set your own reminders.

October: The month most people ignore

October 15: Tax extension deadline. If you filed for a six-month extension back in April, this is the hard stop. No more extensions. File by midnight or face late-filing penalties on top of whatever you already owe. According to the IRS, roughly 19 million taxpayers file extensions each year, and a surprising number still scramble at this deadline. If you owe taxes and haven't been making estimated payments, interest has been accumulating since April 15.

October 15: Medicare Annual Enrollment begins. This window runs through December 7 and lets you switch between Original Medicare, Medicare Advantage, and Part D drug plans. If your current plan sent you an Annual Notice of Change (ANOC) letter, read it carefully. Plans can change formularies, drop providers, and raise premiums year to year. CMS data from the 2026 enrollment period shows that beneficiaries who compare plans during open enrollment save an average of $400 to $600 annually on premiums and out-of-pocket costs.

Mid-October: Review your employer benefits packet. Most employers start distributing open enrollment materials around now, even though your enrollment window probably falls in November. Read the packet early. Benefit structures change, and the cost difference between an HDHP and a PPO can run thousands of dollars depending on how you use health care. The 2026 HSA contribution limit is $4,300 for individuals and $8,550 for families, and you need to be in a qualifying HDHP to contribute.

November: Open enrollment season

November 1: ACA Marketplace Open Enrollment starts. If you buy health insurance through HealthCare.gov or a state marketplace, this is your window. The enrollment period runs through mid-January 2027 in most states, but there's an important catch: if you want coverage starting January 1, you need to select a plan by December 15. After that date, coverage won't kick in until February.

Seven states run their own exchanges with different deadlines. California, New Jersey, New York, and Rhode Island extend enrollment through January 31, while Idaho closes on December 15. Check your state exchange if you're not on the federal marketplace.

November (varies by employer): Workplace open enrollment. This is your annual shot at changing health plans, adjusting life insurance, and (this is the one people forget) setting your FSA and HSA elections for 2027. Your 2026 FSA election can't be changed mid-year without a qualifying life event, so think carefully about what you'll actually spend.

A few things worth checking during enrollment: Did your plan change its formulary? Is your doctor still in network? Did your employer add a new benefit you're missing, like a dependent care FSA or legal plan? According to a 2025 SHRM survey, roughly 30% of employees spend less than 30 minutes reviewing their benefits, and many leave employer matches and reimbursements unclaimed.

Throughout November: Start your tax-loss harvesting. If you hold taxable investment accounts, look at positions that are underwater. Selling them before December 31 lets you use those losses to offset capital gains and up to $3,000 of ordinary income. Unused losses carry forward indefinitely. Starting in November rather than scrambling in late December gives you time to be deliberate about which positions to sell and what to replace them with, while staying on the right side of the 30-day wash-sale rule.

December: The hard deadlines

December 7: Medicare Annual Enrollment closes. This is a firm cutoff. If you haven't reviewed and switched your Medicare plan by this date, you're locked in for 2027 (with a narrow Medicare Advantage Open Enrollment window in January through March for limited changes). Don't wait until December 6 to log in.

December 15: ACA Marketplace deadline for January 1 coverage. Select your plan by midnight on December 15 if you want your new health coverage to start at the beginning of the year. If you miss this date, you can still enroll through mid-January, but your coverage won't begin until February 1.

December 15: Corporate estimated tax payment. If you run a C corporation, the fourth estimated tax installment for 2026 is due December 15. Individual estimated tax payments follow a different calendar, with the Q4 payment due January 15, 2027.

December 31: Spend your FSA balance. This is the default deadline for most flexible spending accounts. Money left unspent is forfeited, period. Some employers offer a grace period (extending to March 15, 2027) or a carryover (up to $680 into 2027), but your plan can only offer one of these, never both. Check with your HR department now, not on December 28. If you're not sure what to spend on, dental work, new glasses or contacts, and physical therapy copays all qualify.

December 31: Take your Required Minimum Distribution. If you're 73 or older and have a traditional IRA or 401(k), your RMD must be withdrawn by December 31. The penalty for missing this deadline dropped from 50% to 25% under the SECURE 2.0 Act (and down to 10% if corrected within two years), but even the reduced penalty is painful on a five-figure distribution. If you turned 73 in 2026, you technically have until April 1, 2027, for your first RMD, but that means doubling up distributions in 2027 and potentially pushing yourself into a higher tax bracket.

December 31: Complete any Roth conversions. Roth conversions must be finished by year-end. There's no extension. If your income is unusually low in 2026 (maybe you retired mid-year, took a sabbatical, or had a business loss), this could be a good year to convert. The converted amount is taxed as ordinary income, so running the numbers before you convert matters more than the conversion itself.

December 31: Make charitable contributions. Cash donations and gifts of appreciated stock must be completed by year-end to count for your 2026 tax return. If you're over 70 and a half, a qualified charitable distribution (QCD) from your IRA (up to $105,000 in 2026) can satisfy part or all of your RMD while keeping the distribution out of your taxable income. Donor-advised fund contributions also need to be initiated by December 31, though the fund itself can distribute the grants to charities later.

December 31: Max out your 401(k). The 2026 contribution limit is $24,500 (or $32,500 if you're 50 or older, and $35,750 if you're 60 to 63 under the new SECURE 2.0 catch-up). If you haven't been contributing enough throughout the year, check whether your employer allows lump-sum catch-up contributions in your final paycheck. Not all plans do, so ask your benefits administrator now.

The one that sneaks up on everyone

There's one more deadline that doesn't have a fixed date: reviewing your W-4 withholding. Your last opportunity to adjust withholding for 2026 is effectively your final paycheck of the year. If you got a big refund last April, you're giving the government an interest-free loan. If you owed a lot, you might face an underpayment penalty. The IRS Tax Withholding Estimator at irs.gov takes about 15 minutes and is worth the time.

The 2026 standard deduction is $16,100 for single filers and $32,200 for married filing jointly, according to IRS Revenue Procedure 2025-11. If your itemized deductions are close to those thresholds, consider whether bunching deductions (prepaying January's mortgage interest or property taxes in December, for instance) could push you over the line.

Your action plan

I'd set three calendar reminders today. First, one for the week of October 15 covering the tax extension deadline and the start of Medicare enrollment. Second, one for early November to review workplace benefits and ACA options. Third, one for mid-December to handle the cluster of year-end deadlines, including RMDs, Roth conversions, FSA spending, and charitable giving.

Missing any single deadline on this list probably won't wreck your finances. Missing several of them compounds quickly. The common thread is that almost every one of these deadlines is a "do it before midnight or wait a full year" situation. The best time to deal with them is while you're still thinking about them, which is right now.